Solenopsis carolinensis, full face view of a worker (click image to enlarge).
Photo courtesy of http://www.antweb.org/

Solenopsis carolinensis, side view of a worker (click image to enlarge).
Photo courtesy of http://www.antweb.org/

Solenopsis carolinensis, side view of a queen (click image to enlarge).
Photo courtesy of http://www.antweb.org/

Solenopsis carolinensis, side view of a male (click image to enlarge).
Photo courtesy of http://www.antweb.org/

Introduction
The genus Solenopsis includes both the "fire ants", known for their aggressive nature and potent sting, and the minute "thief ants", many of which are lestobiotic subterranaen or arboreal species that are rarely collected. Many species are polygynous.

Solenopsis carolinensis is a common species of thief ant in the Southeast, but due to its similarity to several other species and various taxonomic problems with the group in general, it is very difficult to differentiate it from other thief ant species in the region. Historically, most species of thief ants were all identified as S. molesta. However, as indicated by Thompson (1989), specimens identified as S. molesta in collections and in the literature actually represented several species. For accurate identifications, females and males may be needed, as well as an understanding of nesting habitats. DNA analysis may also help resolve questions.

Identification
Generic level identification of Solenopsis is relatively straight forward, although sizes of workers are greatly variable ranging from approximately 1.0 mm to over 5.0 mm. The genus basically can be characterized by the following: mandible with four teeth (usually), bicarinate clypeus with 0-5 teeth, median part of clypeus with a pair of longitudinal carinae medially or at lateral edges, 10-segmented antenna that terminates in a distinctive 2-segmented club, overall shiny appearance and general lack of or reduced sculpture (when present usually restricted to rugulae or striae on the head, alitrunk, petiole, and postpetiole), lack of propodeal spines or other protuberances on the alitrunk, well developed petiole and postpetiole, and a well developed sting. Workers are either polymorphic (especially in the fire ant group) or monomorphic (especially thief ants). In addition to these characteristics, the thief workers in the thief ant group are minute (usually under 2.0 mm in total length), usually have minute eyes (usually with only 1-5 ommatidia (rarely more than 18, except for S. globularia in our region), and minor funicular segments 2-3 are typically wider than long (usually longer than wide in the fire ant group).

Hybridization is not uncommon among related species in the fire ant group, which can make identification of some species difficult. Identification of thief ants is perhaps even more challenging due to their minute size, similar appearance of workers of one species to another, various taxonomic problems, and lack of complete knowledge of all castes.

Worker: Minute: 1.32-1.48 mm TL (measurements from Pacheco 2007). Concolorous yellow. Head longer than wide, subquadrate. Lateral clypeal teeth angulate; extralateral teeth absent. Length of segments 3-8 of the funiculus shorter or about equal to the distance between the frontal carinae (length less than 0.1 mm, rarely longer). Eyes small with 3-5 facets. Alitrunk smooth and shiny. Metapleuron with weak horizontal striae. Petiolar node subtriangular and rounded, with a ventral tooth. Postpetiolar node globose, lacking tooth. Pilosity is abundan on entire body; hairs on posterior tibia semierect.

Female: Medium sized (4.08-4.20 mm TL). Concolorous yellow. Head longer than wide, subquadrate. Lateral clypeal teeth angulate; extralateral teeth absent. Eye black, large (about 0.25 mm in diameter). Ocellae dark; medial ocellus large. Pronotum with coarse punctation, but smooth and shiny between punctures. Mesopleuron lacking sculpture. Lower metapleuron with distinct horizontal striae. Petiole wider than postpetiole, and with distinct ventral flange.

Male: Size: 2.88-3.00 mm TL. Bicolored with brownish head and gaster, yellowish brown alitrunk, petiole, and postpetiole, and yellow to yellowish brown antennae and legs. Head wider than long, shiny and smooth. Eye black and large (approximately 2.40 mm in diameter). Clypeus convex and lacking teeth or other protuberances. Frontal lobes widely separated. Alitrunk shiny and smooth. Petiole wider than postpetiole (in lateral view) and with small ventral flange. Postpetiole wider than petiole in dorsal view and lacking ventral tooth or flange. Pilosity abundant over entire body and appendages.

Workers of this species are remarkably difficult to differentiate from S. texana, S. molesta, S. abdita, and several other species of thief ants. Solenopsis carolinensis differs from S. molesta by its minor funicular segments being shorter (length of segments 3-8 of the funiculus shorter or about equal to the distance between the frontal carinae - less than 0.1 mm; whereas, in S. molesta the minor funicular segments are are usually greater than 1.2 mm. The females of these species differ with S. carolinensis queens being smaller (4.1-4.3 mm TL) and S. molesta queens being larger (4.7-54 mm TL). Males of S. carolinensis are bicolored and males of S. molesta are dark brown to blackish with pale appendages. Solenopsis carolinensis is most easily separated from S. texana and S. abdita by the queens: S. carolinensis queens are yellow and have large eyes (0.25 mm in diameter); S. texana queens are yellowish brown to medium brown and have small eyes (0.16 mm in diameter); and S. abdita queens are dark brown to blackish with small eyes (1.7-18 mm in diameter). Workers of S. carolinensis differ by having erect to semierect hairs on the posterior femur (appressed in S. texana), and they differ from workers of S. abdita by having erect hairs on the scapes (appressed in S. abdita).

Biology and Economic Importance
Because thief ants have been incorrectly identified for so long, it is difficult to truly understand the economic importance of this species. Based on my collections, I have found that S. carolinensis seems to be very common and abundant in natural habitats; whereas, S. molesta may be relegated to more disturbed habitats, or areas associated with human habitation. We routinely collect S. carolinensis in forests in the Southeast, although, this species appears to be replaced in sandy soils with S. abdita. Solenopsis carolinensis commonly nests in bark of pine trees. In fact, this species is easily collected by simply pulling back pine bark at the bases of pines (especially Pinus taeda), where colonies may be revealed.

Distribution
AL, AR, FL, GA, LA, MS, NC, TN

Literature Cited
Pacheco, J. A. 2007. The New World ants of the genus Solenopsis (Hymenoptera: Formicidae). PhD dissertation, The University of Texas at El Paso. i-xxi +543 pp.

Thompson, C. R. 1989. The thief ants, Solenopsis molesta group, of Florida (Hymenoptera: Formicidae). Florida Entomologist 72(2): 268-283.

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